A number of commercial ethylene oxide processes utilize a tube sheet reactor for converting ethylene to ethylene oxide. This fixed bed reactor typically utilizes a silver-based catalyst which has been supported on a porous support and which is typically promoted with an alkali metal promoter. The shell side of the ethylene oxide reactor typically utilizes a high temperature coolant to remove the heat generated by the oxidation reaction. Under operating conditions a chlorohydrocarbon moderator is utilized to control the oxidation reaction.
The usual practice for starting up new silver-based ethylene oxide catalysts in a commercial plant is to first add ethylene and diluent gas, then slowly introduce oxygen to get the reaction started, then to gradually introduce chlorohydrocarbon moderator to control the reaction after it is producing enough heat to become self-sustaining. For the traditional silver-based, alkali metal promoted supported catalyst, the chlorohydrocarbon moderator serves to decrease the activity (i.e., raise the temperature required to obtain a given conversion level) while increasing selectivity to ethylene oxide. When utilizing conventional alkali-promoted, supported silver catalysts, the catalysts are very active at normal stat-up temperatures. Chlorohydrocarbon moderator levels are introduced after start-up to control the high catalyst activity to reduce the conversion level, and to prevent a "run away".